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3 York Centrifugal Chiller Staging

Hi All,
I'm a Refrig and Burner tech, working at a plant type facility and am no expert on chillers, but we're having an issue regarding staging of 3 chillers, and I thought I'd post my question here to get your opinion. We have 3 York chillers, and the maintenance and repairs are done by York. The issue we have is that the way the units are setup, when there is a low load on the system, instead of running 1 chiller at 75%, they'll run all 3 at 25%, and when load is satisfied, all 3 shut down, then when there is a load again they'll start running again distributing the load across all 3, instead of having a lead/lag situation where 1 chiller carries the load until the next one is required, and so on. Before the chillers shut down when load is satisfied and when they startup, we hear a lot of surging and groaning from them. Despite having brought this up with the chiller techs several times, he keeps telling us this is normal, and that distributing the load across all 3 is the way they're supposed to run, but this still doesn't sound right to me. I've always been taught that running equipment in a constant low load situation isn't good, as well as the more frequent stop start situation this seems to cause. Any thoughts?

Stage chillers off system delta. Determine best add-subtract delta's based off your plants needs. Another option is to add-subtract based off running chiller % of capicity. Of course both recommendations need to be tweaked to suit your needs. Im surprised you haven't gotten the York guys involved in this.

Yeah, Dave - I'd like to share some thoughts. You need to get someone that's competent to do your service work, because what you're describing isn't normal, and anyone that's qualified and competent to perform this type work will tell you that. The only way that what you describe would be acceptable is if you have some operating situation in your plant that's so screwed up I can't even think of what it would be, but it would need to be fixed. And I just don't think that's the case.

As has been stated, staging can and should be performed. Whether by FLA, return water temp, or (depending on your controls and system setup) tonnage required, or a combination of any or all of the above doesn't matter. Your service provider is costing you money and unnecessary wear and tear on your equipment if they say otherwise.

The problem is we do not have enough information here yet. Is this a 1 pump, 2 pump or a 3 pump system? Is this a coupled or decoupled system? Are the chillers in series or parallel? Remember chillers require balanced water flow and disturbing this flow WILL cause problems in the evaporator. The chiller plant has a requirment for flow AND the building has a requirment for flow or whatever the load is. If you upset that balance you will be defeating your purpose, or making your chillers run worse. Multi chiller plants are harder to run depending on what pumping scheme you are dealing with. If you reduce the number of chiller on line you may not have enough water flow to make it to the end of the loop or you will starve all the AHU's by dropping the flow by 66% running one chiller. You may still cool but your AHU'S need enough chilled water to bring the supply air temperature below 61 degf to remove humidity. When your air temp rises above that point humidity control has been lost, which is a way to bring in sick building syndrome.

Anther item to look at is cold CHWR this is the one thing that kills multi chiller plants, and makes more tonnage run than is required with simple math. A low delta plant is forcing more tonage to run to satisfy flow requirments. CHWR needs to be in the 50's as close to design as possible. If the CHWS is in the 40's your load has problems manifested in the plant

Well it depends

As others here mentioned there are several unknowns pertaining to the physical plant's actual condition and layout. How well do the chillers perform based on reliability and a kw per ton standard?
This is where the process should start. Then if the pumping strategies are sound, cooling tower temp control is reset and maintained, water treatment is good, barrels are clean, no ancillary equipment and or connected load equipment is causing (mood swings) the chiller plant must endure to remain online at this point you are probably ready to tackle chiller plant stage control.
The most efficient chiller plant stage control will employ adding chiller tonnage and it's support equipment based on maintaining the supply temperature required of the connect load.
The system will subtract tonnage based on the running chiller(s) actual motor load.
These are a few of the things that will be in the makings of a penny-pinching chiller plant.
Always remember as with any refrigeration system things are always in a state of flux.
Changing one little thing in a chiller plant can have catastrophic effects on another.